The present invention relates to a process for the preparation of trifluoroethanol. It relates more particularly to a process for the preparation of trifluoroethanol by the hydrogenation of trifluoroacetic acid.
It is known to prepare perfluoroalkylated alcohols from the corresponding acids by at least two techniques; the first involves acids in the vapor phase, the second involves acids in the liquid phase.
According to the first technique described, for example, in U.S. Pat. Nos. 3,390,191 and 4,273,947, the perfluoroalkylated carboxylic acid in the gaseous form is put into contact with a chrome- or copper-based catalyst (U.S. Pat. No. 3,390,191) or a rhodium- or iridium-based catalyst (U.S. Pat. No. 4,273,947) at a temperature of between the boiling point of the acid and 400.degree. C. The two patents describe only the hydrogenation of trifluoroacetic acid, and the yields given are often low; 1.4% according to U.S. Pat. No. 4,273,947 and 37% according to U.S. Pat. No. 3,390,191. A continuous process for hydrogenation of trifluoroacetic acid, such as described in U.S. Pat. No. 4,396,784, is also known, which consists in hydrogenating this acid on a rhenium--based catalyst deposited in a weight ratio of 3.3% on a fluorinated alumina. The process is carried out at a temperature of 280.degree. C. at a pressure of 20 bars.
The chemical industry prefers to use processes in the liquid phase because of the low yields, the temperatures required to obtain these yields and the technology required for the implementation of reactions in the gaseous phase.
The second technique for the preparation of trifluoroethanol in the liquid phase had been described, in particular, in U.S. Pat. Nos. 3,663,629 and 4,273,947. According to these patents perfluoroalkylcarboxylic acid and hydrogen are put in contact in an autoclave with a catalyst chosen from ruthenium (U.S. Pat. No. 3,663,629) and rhodium or iridium (U.S. Pat. No. 4,273,947). The hydrogen pressure used is very high in U.S. Pat. No. 3,663,629, preferably between 40 and 400 bars. In U.S. Pat. No. 4,273,947 the hydrogen pressure is much lower, preferably between 5 and 15 bars. In either patent, the quantity by volume of acid introduced, calculated with respect to the total volume of the reactor, which is 1%, would not allow industrial production. All of the processes for hydrogenation by a liquid phase technique are difficult to extrapolate to the industrial scale since they are always discontinuous processes which necessitate emptying and cleaning times for the reactors, thereby increasing the price of the final product.
All these documents prove that the solution to the problem which the industry seeks to resolve, that is, to have a continuous process for the preparation of trifluoroethanol from trifluoroacetic acid in the liquid phase using a catalyst which is readily available and with excellent yields, is not obvious.